Network manageable advanced gas sensor apparatus and method

ABSTRACT

Mechanical, electronic, algorithmic, and computer network facets are combined to create a highly integrated advanced gas sensor. A sensor is integrated into switchgear housings. These sensors integrated into high voltage switchgear products, deployed by electric utility end users in replacement and expansion cycles, function to detect and mitigate atmospheric pollution caused by leaking SF 6 . As its associated gas insulated tank is charged with 10 to 350 lbs. of SF 6 , each gas sensor monitors its local cache of gas, accurately sensing and computing fractional percentage losses (emissions) and gains (maintenance replacement) in SF 6  mass, storing data in onboard data logs, and communicating data when triggered by detection events or in response to remote requests over a hierarchical communications network, a process that continues without labor until a fractional leak is automatically detected and reported creating the opportunity for early leak mitigation.

This application is a continuation of prior filed U.S. patent application Ser. No. 13/568,108, filed Aug. 6, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/515,834, filed on Aug. 5, 2011, and Application No. 61/542,261, filed on Oct. 2, 2011, the entire contents of all of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The field of invention is the field of intelligent gas sensors with the capability to measure the pressure and temperature of one or more target gas substances contained in a known volume and to compute the mass of gas so contained as it varies in time due to additions or losses. The invention is also in the field of intelligent networked sensor nodes that exchange sensor information and sensor configuration and control information over communication networks. The field of invention also includes sensors that measure time-varying environmental conditions such as ambient temperature, atmospheric pressure, ambient light conditions, ambient sound levels, as well as various electrical conditions of equipment adjacent systems including AC and DC voltages and currents. The invention also comprises the field of dielectric gas sensors and gas leakage sensors.

BACKGROUND OF THE INVENTION

There is a clear need for a low cost, network manageable, advanced gas sensor for sulfur hexafluoride gas (SF₆) used in high voltage electric switchgear. SF₆ plays a crucial arc-suppression role in this equipment. An expensive commodity and a potent greenhouse gas (GWP 23,900 times that of CO2), SF₆ lost through leakage is a costly problem justifying an effective monitoring system. The instant invention appreciates the application requirements and the sensor and communications network technologies required to meet them. The invention further supports security aspects that are paramount and tolerates the outdoor substation application environment which is challenging.

Worldwide, of 7 million kg of SF₆ produced annually, most (˜75% or 5.5 metric tones per annum) is used for electric power equipment. Consequences for the environment and cost implications for electrical energy producers and users are clearly conveyed. Lower-impact, lower-cost alternatives to SF₆, though sought, are not found. Techniques for estimating emissions have been based predominately upon indirect, mass-balance accounting methods that are costly and error-prone. Trials using expensive equipment (e.g. IR camera) combined with substantial labor have nonetheless shown that environmental impacts and gas expense arising from leakage are significant and can be reduced.

Presently, SF₆ contributes 3% CO₂-equivalent emissions. As global electric usage (3×10⁶ Wh/capita) ascends to U.S. levels (1.3×10⁷ Wh/capita), global generation increases 5-fold. While CO₂ emission per kWh generated must surely decrease, SF₆ emissions will scale with distribution. Switchgear equipment manufacturers and utilities need a low cost, network manageable, advanced gas sensor to achieve reductions in SF₆ emissions per kWh.

All electric producers and users benefit. The instant invention targets economical, distributed sensor technology that can be applied worldwide to achieve a 100-fold reduction in emissions rate—a tremendous opportunity for the environment and economies worldwide.

References Cited and incorporated herein by reference hereto in their entirety follow:

[1] United States Environmental Protection Agency, “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008”, Washington, D.C., Apr. 15, 2010.

[2] Debra Knopman, Katie Smythe, “2004-2006 SF6 Data Summary”, PM-2327-NEMA, June, 2007, Prepared for the National Electrical Manufacturers Association.

[3] United States Environmental Protection Agency, “SF6 Emission Reduction Partnership for Electric Power Systems—2007 Annual Report”, Washington, D.C., December, 2008.

[4] Jos Olivier, Joost Bakker, Jan Willem Wouda, Rainer Bitsch, and Manfred Maiss, “Global Emission Sources of Greenhouse Gas Emissions from Industrial Processes: SF6”, IPCC Task Force on National Greenhouse Gas Inventories, January, 2003.

[5] L. G. Christophorou, J. K. Olthoff, and D. S. Green, “Gases for Electrical Insulation and Arc Interruption: Possible Present and Future Alternatives to Pure SF6”, NIST Technical Note 1425, November, 1997.

[6] United States Environmental Protection Agency, “Electric Transmission and Distribution Equipment Use—Final Rule: Mandatory Reporting of Greenhouse Gases (40 CFR 98, Subpart DD)”, November, 2010.

[7] Alfieri, M. 2002. “Partner Case Study: Con Edison”, Presented on behalf of Con Edison at the International Conference on SF6 and the Environment: Emission Reduction Strategies. San Diego, Calif., Nov. 21-22, 2002.

[8] Robert Madding and Robert Benson, “Detecting SF6 Insulating Gas Leaks with an IR Imaging Camera”, Electricity Today, pp. 12-15, November/December, 2007.

[9] Jan-Martin Rhiemenier, Sina Wartmann, Marcello Pagnotta, Natalia Makowska, and Xingyu Li, “Update on global SF6 Emissions trends from electrical equipment—Edition 1.1”, Ecofys Germany GmbH, July, 2010.

[10] U.S. Department of Energy, “U.S. Energy Information Administration Electric Power Annual 2009”, Washington, D.C., November, 2010.

[11] WIKA Alexander Wiegand GmbH & Co. KG, “Gas Density Monitor (GDM) with Integrated Gas Density Transmitter, Model 233.52.100 TI”, Klingenberg, Germany, May, 2009.

[12] J. Blackman, M. Averyt, and Z. Taylor, “SF6 Leak Rates from High Voltage Circuit Breakers—U.S. EPA Investigates Potential Greenhouse Gas Emissions Source”, presented at the International Conference on SF6 and the Environment: Electric Power Systems—Partnership Update, Nov. 28, 2006.

[13] General Electric Company, “72.5 kV Circuit Breakers Data Sheet”, Nov. 10, 1999.

[14] General Electric Company, “121 kV Circuit Breakers Data Sheet”, Mar. 1, 2002.

[15] General Electric Company, “145 kV Circuit Breakers Data Sheet”, Nov. 10, 1999.

[17] General Electric Company, “169 kV Circuit Breakers Data Sheet”, Nov. 10, 199.

[18] General Electric Company, “242 kV Circuit Breakers Data Sheet”, Nov. 10, 1999.

[19] General Electric Company, “362 kV Circuit Breakers Data Sheet”, Nov. 10, 1999.

[20] General Electric Company, “550 kV Circuit Breakers Data Sheet”, Nov. 10, 1999.

[21] Solon Manufacturing Company, “2TC, SF Gas Density Switch, Intrinsic Gauge Design”, Chardon, Ohio.

[21] Giancarlo Scalabrin, Luigi Bettio, Paolo Marchi, and Paolo Stringari, “A Fundamental Equation of State for Sulfur Hexafluoride (SF6) in Extended Equation of State Format”, JPCRD 36(2) pp. 617-662, 2007.

[22] Maryland Department of the Environment, “Maryland CO2 Budget Trading Program, COMAR 26.09.03”, Baltimore, Md., August, 2009.

[23] California Environmental Protection Agency, Air Resources Board, “Proposed Regulation Order: Regulation for Reducing Sulfur Hexafluoride Emissions from Gas Insulated Switchgear”, Sacramento, Calif., Jan. 7, 2010.

[24] United Nations Framework Convention on Climate Change, “SF6 Emission Reductions in Electrical Grids”, Bonn, Germany, Sep. 29, 2006.

[25] United States Department of the Interior Bureau of Reclamation, “Management and Safe Handling Procedures for Sulfur Hexaflouride (SF6) Gas”, March, 2004.

Each of the foregoing references is included with an information disclosure statement filed contemporaneously with the filing of the instant patent application.

SUMMARY OF THE INVENTION

Although this patent application emphasizes use of the invention for sensing SF₆ in electric breaker applications, it is an important goal of the invention to be readily adaptable to many different gases and gas mixtures used in a broad range of processes.

This invention combines the mechanical, electronic, algorithmic, and network facets needed to create a technology platform for highly integrated gas sensors. These sensors are of great value to electric utility companies and therefore to the manufacturers of equipment used by the utilities. A sensor will be usefully integrated into each gas insulated tank of each breaker and switch unit manufactured (tens of thousands of sensors). These sensors integrated into high voltage switchgear products, deployed by electric utility end users in replacement and expansion cycles, function to detect and mitigate atmospheric pollution caused by leaking SF₆. As its associated gas insulated tank is charged with 10 to 350 lbs. of SF₆, each gas sensor monitors its local cache of gas, accurately sensing and computing fractional percentage losses (emissions) and gains (maintenance replacement) in SF₆ mass, storing data in onboard data logs, and communicating data when triggered by detection events or in response to remote requests over a hierarchical communications network, a process that continues without labor until a fractional leak is automatically detected and reported creating the opportunity for early leak mitigation. Sensors also detect and log repair events including the addition of gas made to replace losses, thus closing the overall monitoring and mitigation loop.

Although a variety of devices and systems for monitoring and measuring aspects of SF₆ gas in laboratory and field settings are currently described in the research and trade literature, none represent a fully integrated, economical, network interface-able component for automatically monitoring SF₆ gas trends in real-time on a tank-by-tank globally distributed basis. Approaches based upon IR imaging such as EPRI and FUR devices are expensive in both equipment and labor and therefore find use monitoring for gas leaks only on a spot versus continuous basis.

High voltage breakers and gas insulated switchgear (GIS) require their SF₆ content to be carefully monitored and controlled. Arc-suppression safety becomes an issue when gas supply is insufficient. Overpressure is problematic with excess gas levels. Determining that gas levels are in the desired range is generally achieved by gas density estimates which in turn are generally derived from gas pressure measurements appropriately compensated for temperature variations. The well known ideal gas law provides a simple model which conveys the concept:

$\begin{matrix} {{pV} = {{{nRT}\therefore\frac{n}{V}} = \frac{p}{RT}}} & (1) \end{matrix}$

Where P is gas pressure in the system, V is the volume of gas which is fixed by the equipment's rigid tank, R is a constant, T is temperature, and n is the mass quantity of gas. With V and R constant, measuring P and T determines n/V, the gas density.

Two types of products have been developed which address the gas content control task. One type, which may generally be referred to as a gas density “monitor”, detects gas density by comparing relatively few thresholds such as: a) high limit, b) nominal limit, c) low limit, and d) low lockout limit. This allows the user to resolve gas density into one of five broad bins: 1) above a, 2) between a and b, 3) between b and c, 4) between c and d, and 5) below d. As illustrated in table 1, while this information is sufficient to enforce the above mentioned safety functions, it falls short of the resolution needed for meaningful emissions mitigation. Manufacturers producing gas density monitor-type products include Solon Manufacturing, Wika, and Comde. In general, these products, unlike IR cameras, are relatively low cost (under $1k USD), of a simple and robust design, well accepted in the marketplace, and therefore in wide use.

A second type of product for gas control applications may generally be referred to as a gas density “transmitter”. This variant measures gas parameters including pressure and temperature to higher resolution, incorporates electronics to derive a temperature compensated density from those measurements, and transmits a density proportional electrical output such as the standard 4-20 mA current loop. These devices, newer to the market, tend to be substantially more complex and costly. The higher resolution density measurement is a step closer to being useful for meaningful emissions detection and mitigation, but a substantial amount of additional functionality must be added externally by the user to interpret the density signal, track and log trends, and communicate decisive information over the user's management network.

Accounting for the impact of temperature variation is of course an important aspect of accurate gas density and therefore accurate gas mass predictions. The operating temperature range for breakers of table 1 is uniformly −40° C. to 40° C. At a nominal pressure of 75 psig at 20° C., this temperature variation corresponds to a −15 psi to +5 psi variation in pressure. Under equilibrium conditions, the temperature compensation is straight forward. However, temperature is rarely expected to be “at equilibrium” in the case of breakers and Gas Insulated Switch (GIS) equipment deployed in outdoor environments across the land. A host of factors including sun, wind, precipitation, and weather in general will drive short-term and diurnal temperature variations which in turn will create temperature gradients across tanks of SF₆ gas. Applying the necessary algorithms to effectively compensate temperature dynamics to achieve the desired detection accuracies yet avoid false alarms is a major accomplishment of this invention.

In summary, achieving SF₆ detection and mitigation efficiency several orders of magnitude better that current practice, to maintain or improve on current levels of leakage in the face of anticipated global electrical consumption increases, according to the foregoing analyses, requires a 100-fold improvement which in turn implies gas sensor detection sensitivities of 0.5 kg to 1.0 kg reliably achieved over dynamic thermal conditions. The instant invention, achieving the aforementioned detection sensitivity and combining network communications to trigger early service mitigation, brings the 100-fold improvement goal within reach.

Practically speaking, the invention represents an advanced gas sensor that both leverages the advantages of existing technology and applies innovations to overcome its shortcomings with respect to the SF₆ emissions mitigation application. It can be globally deployed on breakers and GIS equipment, will accurately track gas additions and losses in real-time, and will be readily integrated into a broad network management infrastructure enabling cost-effective emissions mitigation.

The economic and ecological importance of improved SF₆ gas management has been emphasized. In real terms, each of 6.8 billion humans on earth is a stakeholder. The future of his environment, the quality and cost of his electricity, and the cost of all other goods and services he covets (that rely upon electricity) are at stake.

The most immediate beneficiaries of this invention and its technology will be companies that manufacture and sell the advanced sensors it enables. This invention and technology is conceived to be low ingredient cost and designed for manufacturability from inception. Inherently software configurable, it supports flexible optioning and extensible functionality. As to their customers, advanced gas sensor component manufacturers will enjoy the same growing market now shared by conventional gas density switch manufacturers, namely breaker and GIS switchgear OEMs, electric utilities, and other electric substation designers and operators. For example, a manufacturer of gas density switches in North American markets, estimates annual sales over 10,000 units with significant market growth. For the customers' sakes, this invention and technology is conceived to support the surgical detection, tracking, and mitigation of SF₆ loss through equipment leakage with products that represent low component and operating cost burdens to the user. The economic benefits are manifold:

-   -   Gas expense savings (demand for SF₆ and therefore gas costs,         already ˜$10/lb, is increasing)     -   Direct process data captured automatically inexpensively         demonstrates regulatory compliance, compared to costly, complex,         and error prone mass balance procedural alternatives     -   Avoidance of regulatory fines for emissions; and,     -   Capture of offset credits

What is the market size for customers that desire these benefits? Based upon a weighted, average nameplate SF₆ capacity of 73 kg, and considering global annual SF₆ utilization for electric equipment of 5,500 metric tons, and assuming 3 pole tanks per breaker, one can estimate a global population of equipment increasing at approximately 200,000 tanks per year. Assuming this corresponds to a growth rate of 5%, the global established market can be inferred to be approximately 4 million tanks. This is the immediate market for my sensor invention in the upgrade space. Please see, J. Blackman, M. Averyt, and Z. Taylor, “SF6 Leak Rates from High Voltage Circuit Breakers—U.S. EPA Investigates Potential Greenhouse Gas Emissions Source”, presented at the International Conference on SF6 and the Environment: Electric Power Systems—Partnership Update, Nov. 28, 2006 and incorporated here by reference in its entirety and also submitted herewith through an information disclosure statement.

This invention is conceived to be market friendly, utilizing a mechanical bellows technology and form factor well entrenched in the present market. Flexible network interface functionality renders this sensor easy to integrate in the user's network management system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the gas sensor apparatus.

FIG. 2 is a perspective view of the gas sensor apparatus with covering and housing removed.

FIG. 3 is a perspective view of the gas sensor apparatus similar to FIG. 2 with the pushbutton removed.

FIG. 4 is a perspective view of the gas sensor apparatus similar to FIG. 3 with the temperature sensors and their mounting plate removed.

FIG. 5 is a perspective view of the gas sensor apparatus similar to FIG. 4 with the processor printed circuit board removed.

FIG. 6 is a left side view of the gas sensor apparatus of FIG. 5.

FIG. 6A is an enlarged portion of FIG. 6.

FIG. 6B is a right side view of the gas sensor apparatus of FIG. 5.

FIG. 7 is a top view of the of the gas sensor apparatus as illustrated in FIG. 6 with the printed circuit board removed illustrating the lever and switch.

FIG. 8 is a bottom view of the printed circuit board.

FIG. 8A is a bottom perspective view of the printed circuit board.

FIG. 9 is a bottom perspective view of the gas sensor apparatus.

FIG. 10 is a front view of the sensor internal components.

FIG. 10A is front view of the gas sensor apparatus internal components with the riser cutaway illustrating the bellows.

FIG. 10B is an enlargement of a portion of FIG. 10A.

FIG. 11 is a top view of gas sensor apparatus internal components.

FIG. 11A is a cross-section of FIG. 11.

FIG. 12 is a top view of the gas sensor apparatus internal components.

FIG. 12A is a cross-sectional view of the gas sensor apparatus of FIG. 12.

FIG. 13 is a hardware block diagram.

FIG. 14 is a processing block diagram.

FIG. 15 is a graph lever position as a function of gas pressure at 25° C.

FIG. 16 is a normalized sensor response as a function of lever position.

FIG. 17 is a graph of pressure compensation required as a function of temperature.

FIG. 18 is a graph of temperature of normalized temperature sensor response.

DESCRIPTION OF THE INVENTION

As stated above, the instant invention supports a 100-fold reduction in gas emissions. What does this imply for gas density measurement requirements? To address this question, begin by considering that, at a temperature of 20° C., the operating pressure for the breakers of table 1 ranges from 64 psig to 82 psig, a span of 18 psi.

Table 1 also gives the nominal gas mass change attributable to pressure change for each breaker under the aforementioned isothermal conditions. The function is simply proportional to the differential tank volume of the various breakers given the isothermal assumption. As expected, the largest tank represents a worst case requirement for mass sensing resolution since smaller pressure changes accompany larger gas losses (large mass changes). In general, larger tanks will require higher resolution measurements to detect unit changes in gas mass.

TABLE 1 Breaker Model Designation HS HP-1 HP-2 HP-3 HPI-1 HPI-2 Rated 72.5 145 169 242 345 550 Maximum Voltage (kV) Interrupting 31.5 40 40 40 362 40 Current Rating (kA) Tank Volume 0.151 0.561 0.732 1.171 3.367 3.542 (cubic meters) SF6 weight at 5.2 19.1 24.9 39.9 114.8 120.7 fill pressure (kg) SF6 weight at 4.7 17.4 22.7 36.3 104.3 109.8 nominal (kg) SF6 weight at 4.3 16.0 20.9 33.4 96.1 101.1 alarm (kg) SF6 weight at 4.0 14.8 19.3 31.0 89.0 93.6 lockout (kg) SF6 Emission 0.8 3.1 4.1 6.5 18.7 19.7 between fill and alarm (kg) SF6 mass per 0.063 0.232 0.302 0.484 1.391 1.464 unit pressure (kg/psi) Distribution 50% 14% 13% 15% 6% 2% frequency [12]

Table 1—Representative gas insulated breakers with OEM recommended SF6 fill conditions. If filled to just below fill capacity, breaker type HPI-2 would emit 19.7 kg of SF6 before the alarm threshold would trigger. Typically the so-called “nameplate capacity” will be three times larger than the above tank capacity since the breaker comprises three phases each with its individual tank The approximate frequency with which various sizes occur in practice is attributable from Blackman.

Now the question arises, what is the magnitude of gas loss one needs to begin detecting? SF6 emission rates studied by various methods to date appear to place gas emissions in the range of 5% to 10% of total nameplate capacity annually. Accounting for frequency of distribution of breakers by voltage rating (and therefore by tank size), the weighted average of the nameplate capacities is approximately 73 kg (remember—3 tanks per breaker typically). In a study of 2,329 breakers by Blackman, 170 (7.3%) were found to be leaking. The amount of gas emitted to atmosphere annually may thereby be estimated at 3.7 kg to 7.3 kg per breaker (5% to 10% of 73kg). The actual leaks arise from the aforementioned 7.3% of the breaker population. Therefore, the average leakage amount per leaking breaker is on the order of 50 kg to 100 kg annually.

The sensor-gas interface mechanism as one component of the advanced gas sensor has many important aspects. The use a mechanical bellows approach is utilized for several reasons. These reasons include the bellow's simplicity, reliability, and broad use in SF₆ gas density switch applications. The use of a mechanical bellows leads to a requirement for detecting and processing mechanical displacement information. Processing the displacement information supports accurate gas pressure inferences.

The advanced gas sensor combines a bellows sensing element with an MCU Electronics module comprising electronics and software for acquiring raw displacement and temperature information and processing these into accurate measurements.

Reliable pressure and temperature readings must ultimately be rendered from raw sensor data. The present invention utilizes an efficient signal processing chain for this purpose. Noise, stability, and other potential problems are thereby identified and overcome.

Processed pressure and temperature readings must be interpreted to predict gas density which in turn predicts gas mass changes in light of known, rigid tank volumes. The process, in isothermal conditions, is relatively straight forward. Under conditions of changing temperature, the process becomes more challenging. Ideal gas law and virial equations with alternative techniques for calculating temperature dependent coefficients form the foundations of the algorithms utilized for this purpose.

As stated earlier, the present invention uses a mechanical bellows approach to gas interface and pressure sensing. The advantages of this choice are described above. Mechanical bellows components are readily available from a variety of sources including Solon Manufacturing of Chardon, OH. Mechanical bellows are widely used in mechanical, gas density monitoring products that enjoy a dominant share of the North American alarm-monitoring market.

The bellows expands under increasing pressure. In the configuration of the embodiments set forth herein, the bellows actuates a rigid coupling to a platen. The platen's starting position and translational gain are simultaneously adjusted with a counter-biasing coil spring. Nominal gain in the range of 0.001″ platen deflection per 1 psi change is typically achieved.

In the mechanical density monitor application, the platen carries bi-metal elements that in turn actuate snap-action micro-switches under conditions of sufficient displacement. The bi-metal elements provide a mechanical temperature compensation mechanism.

Contrastingly, in the instant invention, the platen is adapted to carry displacement sensor components which take the form of reflective surfaces, magnets, and other components supporting displacement detection alternatives. FIG. 16 shows the normalized sensor response of an embodiment which utilizes an infrared reflective object sensor (ROS) and another embodiment that uses a Hall Effect sensor (HES). FIGS. 6, 6A, 9, 10, 10A, 10B, and 12A show the mechanical aspect of embodiments using the reflective object sensor and Hall effect sensor components respectively.

As stated above, the invention targets supporting a 100-fold reduction in gas emissions. In the discussion above, it was deduced that a 100-fold improvement in emissions mitigation implies gas sensor sensitivities of 0.5 kg to 1.0 kg. According to table 1 above, this suggests a differential pressure resolution on the order of 16 psi to 0.35 psi. Recall that the operating span of interest is approximately 18 psi. Thus the required pressure measurement resolution (before correction) is in the range of 1 part in 1.2 to 1 part in 52.7. In digital measurement terms, this corresponds to a 1 bit to 6 bit dynamic range, which is achieved using a microcontroller and 12 bit analog to digital converters.

Core bellows devices, prior to any modification, have been bench tested for displacement response over the pressure range of interest at 20° C. Conventional gauge room equipment was used to measure displacement. Regulated compressed air provided pressure actuation. Pressure gradients in both directions have been utilized to quantify hysteresis, and several runs are made to assess short-term repeatability. Analysis of data captured in these tests was analyzed and definitively demonstrates the bellows fitness for the application in this invention as shown hereinbelow.

A microcontroller 1301 is used to perform displacement sensing and temperature sensing, and to communicate raw data to the other controller functions (via asynchronous serial communications initially). FIG. 13 is a simplified block diagram of the MCU 1301 and its interactions with the other elements of the invention. The Hi-RES transducer 1310 can optionally be the aforementioned infrared reflective object sensor (ROS), Hall Effect sensor (HES), or other displacement transducer. The temperature probes can be thermistors 507B, 507D, 617B, 617D, thermocouples, RTD, or other suitable temperature transducers. FIG. 13 illustrates, diagrammatically, temperature probes 1308A, 1308B, 1308C, 1308D located within the switchgear housing. Reference numerals 1308A-D indicate, generically, many different types of temperature probes which may be used. FIG. 13 also illustrates the battery 1311, a temperature interface 1307, a displacement interface 1309, as well as a test controller (network manageable controller) 1312, a communication subsystem 1302, an analog to digital controller 1303, and a digital to analog controller 1304, a digital I/O interface subsystem 1305, and a safety limit detection subsystem 1306.

MCU subsystem modularity allows easy substitution of alternative circuits for the powered by battery DISPLACEMENT INTERFACE 1309 and HI-RES TRANSDUCER 1310. The MCU 1301 monitors battery state of charge and computes circuit power consumption as well, an important distinguishing characteristic of circuit and algorithmic alternatives. The FLASH memory based MCU 1301 may be conveniently reprogrammed to adapt to varying sensing requirements. Operating parameters may be programmed and acquired data retrieved over the bidirectional, asynchronous communications interface.

Initial choices for HI-RES TRANSDUCER 1310 used to measure platen 601 displacement include Hall Effect and photo diode/transistor technologies. The optical alternatives comprise both transmission and reflective technologies. Piezo strain gauge and ultrasonic systems are possible as well.

Processing and calibration requirements for rendering accurate pressure readings from displacement data are included. Temperature channels are also logged during operation. All data generated by the displacement and temperature measurement blocks is forwarded to the Temperature and Pressure Processing blocks. The invention covers the operating and temperature ranges of interest. Temperature gradient and leak rate tests are also satisfied. The invention includes the signal processing necessary for rendering reliable pressure and temperature readings from raw displacement and temperature sensor data.

FIG. 14 is a processing block diagram 1400 which illustrates the general topology for processing displacement data. A similar signal chain is utilized for temperature data. The order of the functions utilized is based upon the characteristics of the raw data and the desired resolution and accuracy of the processed readings. Reference numeral 1401 signifies raw conversions from displacement and temperature subsystems which are input into a system which enhances the signal to noise ratio. Reference numeral 1403 signifies a system which linearizes the displacement to pressure calculation. Reference numeral pressure calibration signifies a pressure calibration system and reference numeral 1405 signifies a digital filtering subsystem. Reference numeral 1406 signifies signal processing parameters which are included in the signal to noise ration subsystem and the linearization of displacement into pressure. Reference numeral 1407 signifies pressure readings output to gas mass calculations and user interfaces.

FIG. 15 is a graph lever position 1500 as a function of gas pressure at 25° C. Reference numeral 1501 indicates the lever response from 0 psig to 60 psig. It will be noticed that line 1501 represents the displacement of the lever with respect over pressure range of 0 to 60 psig and with the lever acting against the ball nose spring plunger. Reference numeral 1502 is a line on the graph of the lever position from 60 to 100 psig for the coil spring 608 and bimetal hinge 708. Reference numeral 1503 is a particular lever position of 0.026″ corresponding to a pressure 1504 of 81 psig.

FIG. 16 is a normalized sensor response 1600 as a function of lever position at 25° C. FIG. 16 is a normalized sensor response 1600 as a function of lever position. Reference numeral 1601 is the response of reflective object sensor and reference numeral 1602 is the response of Hall effect sensor (HES). Reference numeral 1603 is a particular HES response of 0.55 corresponding to a particular lever position 1604 of 0.026″.

N.B. Calibration is achieved entirely using digital techniques to determine coefficients stored onboard in nonvolatile memory. Use of precision or adjustable components is avoided in favor of standard tolerance, inexpensive, high stability components.

An equation of state model is required to compute the target gas density from calibrated temperature and pressure data. A first order Gas Density Model is used and provides satisfactory results in many cases.

The well known virial form set forth below as equation 2 utilizing coefficient functions for SF₆ selected from various perspectives is an alternative embodiment:

$\begin{matrix} {\frac{pV}{nRT} = {1 + {{B(T)}\frac{n}{V}} + {{C(T)}\frac{n^{2}}{V^{2}}} + \cdots}} & (2) \end{matrix}$

Where p, V, n, R, and T have their usual meanings in the ideal gas law, and B(T) and C(T) are the second and third virial coefficients respectively, each non-linear functions of temperature T.

This step further draws upon recent work by Scalabrin describing a computationally efficient neural network technique for computing coefficients in a certain form of state equation.

It is an important aspect of the instant invention to use a micro-power microcontroller platform to sense gas density to sufficient accuracy to discern 0.5 kg emission events under a range of conditions of interest for the largest tank volumes expected.

FIG. 1 is a perspective view 100 of the gas sensor apparatus. Cover 101 and liquid tight pushbutton 102 are shown in FIG. 1. Cover 101 is affixed to housing 104 by cover retaining screws 103. Sensor connector 105 provides communications between the gas sensor apparatus and the exterior of the switchgear cabinet. Power to the apparatus is also supplied through the connector pins 105A. Connector nut 105B affixes the connector to the housing 104. Manifold block 106 includes a first gas port 106B for admission of gas to the gas sensor apparatus. Manifold block bolt hole 106A includes bolts which secure the manifold in place. A display deadfront 109 (display cover) and gasket 107A are illustrated.

FIG. 2 is a perspective view 200 of the gas sensor apparatus with covering 107 and housing 104 removed. Liquid tight pushbutton 102 when depressed provides a temperature compensated pressure readout. Pushbutton cable 202 and connector 203 enables electrical communication between the pushbutton and the electronics on board the gas sensor apparatus. Connector 203 interconnects with processor PCB pushbutton connector 204. Processor printed circuit board 205 is illustrated in FIG. 2.

FIG. 3 is a perspective view 300 of the gas sensor apparatus similar to FIG. 2 with the pushbutton removed. Perspective view 300 of the sensor with pushbutton removed. Display printed circuit board 301 is illustrated as being mounted to the processor printed circuit board 205 using a standoff (spacer 303) and screw 302. Display digits 304 communicate a temperature compensated pressure readout (display). In the approximate middle of the printed circuit board 301, are processor printed circuit board connectors. The display printed circuit board includes a coil spring clearance hole 306.

FIG. 4 is a perspective view 400 of the gas sensor apparatus similar to FIG. 3 with the temperature sensors and the mounting plate for the temperature sensors removed. The temperature sensors 1308A-D, are best viewed, diagramatically in FIG. 13. The temperature sensors will be located in the switchgear compartment or housing in various places so as to obtain accurate temperature readings of the gas being measured. A typical gas used in switchgear is sulfur hexafluoride gas (SF₆). SF₆ plays a crucial arc-suppression role in this equipment. Other gases may be used in the switchgear. Further, this invention is equally applicable to the determination of loss of any gas from any containment structure. As described in further detail hereinbelow, the loss of gas is determined by a change in the temperature compensated pressure.

Referring to FIGS. 11, 11A, 12, and 12A, some of the important internal elements of the invention are disclosed. FIG. 11 is a top view 1100 of gas sensor apparatus internal components. FIG. 11A is a cross-section view 110A of FIG. 11. FIG. 12 is a top view 1200 of the gas sensor apparatus internal components. FIG. 12A is a cross-sectional view 1200A of the gas sensor apparatus of FIG. 12. Switch printed circuit board 501 includes a microcontroller unit 1103, 1301. Base plate 602 is affixed to the adapter flange 604 by unnumbered screws. Lever 601 pivots about a pivot portion (unnumbered) of the coupling 1002 of the bellows 1003. Stabilizers 1104 of the coupling 1002 tend to center the coupling 1002 of the bellows as the bellows is raised and lowered in response to pressure within the bellows. Gas port 1101 communicates gas into the bellows 1003. Riser 603, adapter flange 604, base plate 602 provide a foundation for operation of the lever 601. Lever 601 pivots about coupling 1002. Bimetallic strip 708 is affixed to the lever 601 by retaining plate 706. Bimetallic strip 708 is also affixed to an unnumbered block by retaining plate 619. FIG. 7 illustrates the bimetallic strip 708 and notches cut therein for desired performance thereof. The material of the bimetallic strip 708 is not limited in this specification. The bimetallic strip functions to compensate for the influence temperature of the gas has on gas pressure.

One important object of the invention is to determine if gas is being loss from the switchgear. The gas sensor apparatus operates over a wide range of temperature and pressure conditions other than standard temperature and pressure conditions. If pressure of the gas rises, but the mass of the gas within a known volume stays the same (ie no loss occurs), then the apparent pressure in the volume (tank) appears to increase. The bimetallic strip 708, however, adds a downward force on lever 601 to counteract the additional force of the gas within the bellows due to an increase in gas temperature. If pressure of the gas decreases, but the mass of the gas within a known volumes stays the same (ie no loss of gas occurs), then the apparent pressure in the volume (tank) appears to decrease. In a similar manner, an apparent decrease in gas pressure due to a relatively low temperature, is compensated by an upward force on lever 601 to counteract the reduction in force of the gas within the bellows due to a decrease in gas temperature.

A magnet is affixed to the lever 601. A reflective surface is also affixed to the lever 601. A hall effect sensor is applied to the switch printed circuit board 501. A reflective object sensor is affixed to the switch printed circuit board. In FIG. 12A, reference numeral 1102 is being used to denote the magnet and the reflective surface. In FIG. 12A, reference numeral 1103 is being used to denote the hall effect sensor, the reflective object sensor and the processor module.

FIG. 9 is a bottom perspective view 900 of the gas sensor apparatus. Hall effect sensor 609 and reflective object sensor 610 are illustrated in FIG. 9 on the underside of switch printed circuit board 501. Magnet 611 and reflective surface boss 614 are illustrated residing on lever 601. Lever 601 moves vertically with a small amount of pivotal movement as well as can be visualized in FIG. 12. As lever 601 moves, the hall effect sensor 609 and the reflective object sensor 610, detect the movement. Processor 1301 is not visualized in FIG. 9, but it can reside on the underside of printed circuit board 501 as illustrated in FIG. 12A. Alternatively, processor 1301 can be located on the upper or top side of printed circuit board 501.

Referring to FIG. 13, processor 1301 receives temperature inputs from temperature probes within the cabinet and processes the various temperature signals for further evaluation of the pressure information received from the high resolution temperature transducers 1310. Reference numeral 1310 indicates that “OPTION X” temperature transducer(s) may be used. This means that one or both of the hall effect sensor and/or the reflective object sensor may be used in the calculation of movement of the lever. Movement of the lever in combination with use temperature data, determines the gas density. In this patent application, various parameters are expressed by the ideal gas law stated above.

${pV} = {{{nRT}\therefore\frac{n}{V}} = \frac{p}{RT}}$

Where P is gas pressure in the system, V is the volume of gas which is fixed by the equipment's rigid tank, R is a constant, T is temperature, and n is the mass quantity of gas. With V and R constant, measuring P and T determines n/V, the gas density.

The gas sensor apparatus includes switch actuator elements 704 which reside on lever 601 which engage the actuators 618AA of snap action switches 618A-D as illustrated in FIG. 8. FIG. 8 is a bottom view 800 of the printed circuit board 501. Switches 618A, 618B, 618C and 618D protrude downwardly from printed circuit board 501. Each switch includes an actuator 618AA although only one such actuator is labeled with reference numeral 618AA. When the actuator elements 702 engage the actuators 618AA, then contacts within the switch are electrically joined or completed which results in an alarm, warning, or other signal sent to a user. These switch functions include the temperature compensation provided by the bimetallic strip. Hall effect sensor 609, reflective object sensor 610, reflective object sensor phototransistor 610A, and reflective object sensor infrared LED emitter 610B are illustrated in FIG. 8.

FIG. 8A is a bottom perspective view 800A of the switch printed circuit board 501 wherein the sensor connector 105 and the sensor connector contact pin 105A are illustrated along with the printed circuit board 501. Connector support 502 is affixed to PCB flexible circuit element 503. First 507B and second 507D thermistors are illustrated in FIG. 8A as are third 617B and fourth 617D thermistors. First thermistor stalk 507A and second thermistor stalk 507C are illustrated well in FIG. 8A. Third thermistor stalk 617B and fourth thermistor stalk 617C are illustrated well in FIG. 8A.

FIG. 5 is a perspective view 500 of the gas sensor apparatus similar to FIG. 4 with the processor printed circuit board 205 removed. Reference numeral 501 is the switch printed circuit board and reference numeral 502 is the switch printed circuit board connector. Flexible circuit element 503 is interconnects the connector 502 to the switch printed circuit board 501. Screws 504 retain the printed circuit board to the main structure of the apparatus. Switch connections 506 are viewed in FIG. 5 and enable attachment of the snap-action switches from the bottom side of the printed circuit board 501. The bottom side of printed circuit board is best viewed in FIGS. 8 and 8A. Each of the switches 618A-D is actuated by spring loaded metallic actuator elements 704 best viewed in FIG. 7. The spring loaded actuator elements 704 are very slightly bowed depending on the amount of adjustment 702 which bias the elements 704 and, therefore, control the actuation of the switches. The spring loaded elements 704 are affixed to bimetallic hinge retaining plate 706. Still referring to FIG. 7, the reflector 613, the magnet 611, and the spring stud 701 are illustrated.

FIG. 7 is a top view 700 of the of the gas sensor apparatus as illustrated in FIG. 6 with the printed circuit board 501 removed illustrating the lever 601 and switch actuator elements. Manifold block 106, lever 601, base plate 602, coil spring nut 606, spring stud 701, switch PCB mounting bosses 709A-D, 611 magnet, reflective surface 613, bimetal hinge base retaining plate 619, bimetal hinge base retaining plate nut 620, switch actuator element adjuster screws 702, switch actuator elements 704, switch actuator elements flange screw 705, bimetal hinge lever retaining plate 706, bimetal hinge lever retaining plate nut 707 and bimetal hinge 708 are all well illustrated in FIG. 7.

Referring to FIG. 5 again, thermistor 507A, thermistor stalk 507B and thermistor connections 507 are illustrated. Further, the connection 508 for the reflective object sensor and the cutout 509 for the coil spring are shown.

FIG. 6 is a left side view 600 of the gas sensor apparatus of FIG. 5. FIG. 6 illustrates manifold block 106, switch PCB 501, switch PCB processor PCB connector 506, first thermistor stalk 507A, first thermistor 507B, third thermistor stalk 507C, and third thermistor 507D. Lever 601, base plate 602, riser 603, and adapter flange 604 are illustrated in FIG. 6. Second gas port 605, coil spring nut 606, coil spring washer 607 and coil spring 608 are illustrated in FIG. 6 as well. Riser 603 is generally cylindrically shaped and extends from the adapter flange 604 to the base plate 602.

FIG. 6A is an enlarged portion 600A of FIG. 6. FIG. 6A illustrates the lever 601, the Hall effect sensor 609, the reflective object sensor 610, the magnet 611, the magnet boss 612, the reflective surface 613, the reflective surface boss 614, the ball 615, and the ball spring adjuster 616.

FIG. 6B is a right side view 600B of the gas sensor apparatus of FIG. 5. Switches 618A-D are illustrated attached to the switch printed circuit board 501. Switch PCB connector PCB 502 and the switch PCB flexible circuit element 503 are illustrated in FIG. 6B as well. Third 617B and fourth 617D thermistors are illustrated along with their respective stalks 617A, 617C.

FIG. 10 is a front view 1000 of the sensor internal components. Ball 615 is illustrated in FIG. 10 as is switch actuator element adjuster screw boss 1001. FIG. 10A is front view 1000 a of the gas sensor apparatus internal components with the riser cutaway illustrating the bellows 1003. Bellows lever coupling 1002 is illustrated in FIG. 10A in engagement with lever 601. Lever 601 is movable vertically depending on the pressure applied to the bellows and depending on the action of the bimetallic hinge. As shown in FIGS. 10 and 10A, gap 1004 is the distance between the lever 601 and the ball 615, in other words reference numeral 1004 is the lever displacement dimension.

FIG. 10B is an enlargement 100B of a portion of FIG. 10A illustrating the gap 1004 between the lever 601 and the spring loaded ball 615. As illustrated in FIGS. 10, 10A and 10B, vertically movable lever 601 is positioned by virtue of pressure greater than 60 psig and less than 82 psig.

The ideal gas law restated:

$\begin{matrix} {{pV} = {{{nRT}\therefore n} = \frac{pV}{RT}}} & (1) \end{matrix}$

p=absolute pressure (pounds per square inch or psi)

V=volume (cubic meters)

T=temperature (Kelvin)

n=gas quantity in moles (mol)

R=gas constant=1.2095×10⁻³

It should be noted that, p, the pressure in (1) is the absolute pressure (reference to a vacuum) which differs by atmospheric or barometric pressure from the pressure indicated by a typical gauge in atmospheric conditions. This can be stated mathematically as: p=p _(abs) =p _(g) +p _(atm)  (3)

p=p_(abs)=absolute pressure (psi)

pg=gauge pressure (psi)

p_(atm)=atmospheric pressure (psi)

With n the gas quantity in mol known, the mass quantity for a particular gas is derived from its molar weight: m=nM_(m)  (4)

m=gas quantity in grams (g)

n=gas quantity in moles (mol)

M_(m)=molar mass of gas species (g/mol)

A sequence of measurements of gas mass m_(i)=m₁, m₂, . . . m_(j) can be derived using corresponding sequences of pressure p_(i) and temperature T_(i) measurements given only that the volume V, atmospheric pressure p_(atm), and gauge pressure p_(g) corresponding to each point in the sequence are known. A change in gas mass foretells a leak when a measurement m_(j) is less than a measurement m_(k) made sometime earlier (k<j). Conversely, the addition of gas is detected when m_(j) is greater than m_(k). In a non-leaking system, all of the m_(i) will be substantially equal.

Acquiring temperature sequence T_(i) begins by microcontroller 1301 using analog to digital converter 1303 applied to temperature interface 1307 accessing temperature probes 1308A through 1308D to acquire raw sensor measurements. Raw sensor measurements are then converted to accurate temperature readings through a calibration process such as that depicted in FIG. 18 wherein sensor response is converted to temperature in degrees centigrade for each sensor. Centigrade temperatures are converted to requisite absolute temperatures by addition of the offset 273.15 degrees. A point T_(i) can then be recorded as a particular weighted average of the different sensor's derived absolute temperatures. In the preferred embodiment, the temperature sensors 1308A through 1308D correspond to thermistors 507B, 507D, 617B, and 617D.

Acquiring pressure sequence pi is somewhat more involved. It begins again with microcontroller 1301 using analog to digital converter 1303 applied to displacement interface 1309 accessing high resolution displacement transducer 1310 to acquire raw displacement sensor measurements. Unlike temperature measurements, there is no simple transformation of raw displacement measurements to absolute pressure, however. Firstly, a raw displacement sensor measurement is utilized by the MCU to compute calibrated lever displacement dimension according to calibration data such as that depicted in FIG. 16. In a preferred embodiment, the high resolution displacement transducer is the combination of a reflective object sensor 610 in combination with a reflective surface 613. In this case sensor response is calibrated using data such as that of curve 1601. In another embodiment, the high resolution displacement transducer is the combination of Hall Effect sensor 609 in combination with magnet 611. In this case sensor response is calibrated using data such as that of curve 1602.

Once calibrated lever displacement dimension is derived, initial gauge pressure estimate can be computed using secondary calibration data as depicted in FIG. 15. For example, if Hall Effect sensor response is measured to be 0.55 (1603), lever displacement dimension is determined to be 0.026 inch (1604). This lever location 0.026 inch can be transferred to the graph of FIG. 15 (1503) and used to determine an initial gas gauge pressure estimate of 82 psi (1604).

The intrinsic temperature compensation of the lever system comprising bi-metal hinge 708 must now be taken into account. In the absence of the bi-metal element, lever position would simply track temperature variations. For the fixed volume V, gas pressure increases proportional to increasing temperature (and vice versa). With only the resistance of coil spring 608, lever dimension 1004 would increase proportionately with the varying force exerted by bellows 1003. The bi-metal element is conceived to neutralize this temperature induced pressure variation. As temperature increases, the bi-metal exerts approximately equal magnitude equal force directed oppositely to the increased upward force of the bellows with the approximate result that the lever dimension remains constant. The converse occurs as temperature decreases. These mechanics alone allow the mechanism to operate as a low resolution density monitor wherein eventual changes in lever position represent approximate changes in gas mass (as opposed to pressure variations due to temperature), and, for fixed volume V, gas density. With the advent of the microcontroller in the present invention, it is possible to improve accuracy and flexibility of gas monitoring including the electronic measurement of pressure, temperature, gas content, and gas density as explained above.

To complete the derivation of absolute gas pressure p from displacement and temperature sensor measurements, the initial gas gauge pressure estimate as above must itself be compensated for the temperature behavior introduced by the bi-metal element. The appropriate compensation is derived from the data in FIG. 17 using temperature Ti as above. For example, if Ti is 303K corresponding to a temperature of 29.85 C (1703), a temperature compensation of approximately 2 psi is indicated (1704). Therefore, in the current example, a calibrated gauge pressure is computed equal to 82 psi+2 psi equals 84 psi. A reasonable estimate of atmospheric pressure is used based on typical or measured data. An example of a typical value for atmospheric pressure is 14.7 psi. The measurement of absolute pressure p_(i) is computed as the sum of the gas gauge pressure and the atmospheric pressure, 98.7 psi in the example.

To complete the example, given a typical tank volume V of 1 cubic meter, along with a molar mass for SF₆ gas of 146.055 g/mol, the gas mass m_(i) is computed according to (1) and (4) to be 38.62 kg. The entire process is implemented by microcontroller 1301 in combination with the electronic elements of FIG. 13 and is represented in block diagram form in FIG. 14. All data described above is recorded in microcontroller memory including the raw sensor measurements through the final derived measurement sequences T_(i), p_(i), and m_(i).

REFERENCE NUMERALS

-   100 perspective view of sensor -   101 cover -   102 liquid tight pushbutton -   103 cover retaining screw -   104 housing -   105 sensor connector -   105A sensor connector contact pin -   105B connector nut -   106 manifold block -   106A manifold block bolt hole -   106B first gas port -   107 display deadfront -   107A deadfront gasket edge -   200 perspective view of the sensor with cover and housing removed -   202 pushbutton cable -   203 pushbutton cable connector -   204 processor PCB pushbutton connector -   205 processor PCB -   204 processor PCB pushbutton connector -   205 processor PCB -   300 perspective view of the sensor with pushbutton removed -   301 display PCB -   302 display PCB retaining screw -   303 display PCB standoff -   304 display digit -   305 display PCB processor PCB connector -   306 display PCB coil spring clearance hole -   204 processor PCB pushbutton connector -   205 processor PCB -   400 perspective view of the sensor with display PCB removed -   401 processor PCB retaining screw -   402 processor PCB standoff -   403 processor PCB coil spring clearance hole -   404 processor PCB display PCB connector -   405 processor PCB switch PCB connector -   105 sensor connector -   105A sensor connector contact pin -   500 perspective view of the sensor with processor PCB removed -   501 switch PCB -   502 switch PCB connector PCB -   503 switch PCB flexible circuit element -   504 switch PCB retaining screw -   505 switch PCB processor PCB connector -   506 switch connections -   507 thermistor connections -   507A thermistor -   507B thermistor stalk -   508 reflective object sensor connection -   509 switch PCB coil spring and switch adjustment clearance cutout -   507A first thermistor stalk -   507B first thermistor -   507C third thermistor stalk -   507D third thermistor -   600 left side view of the sensor internal components -   600A detail of lever and displacement mechanisms -   600B right side view of the sensor internal components -   600A detail of lever and displacement mechanisms -   601 lever -   602 base plate -   603 riser -   604 adapter flange -   605 second gas port -   606 coil spring nut -   607 coil spring washer -   608 coil spring -   601 lever -   609 Hall effect sensor -   610 reflective object sensor -   611 magnet -   612 magnet boss -   613 reflective surface -   614 reflective surface boss -   615 ball -   616 ball spring adjuster -   617A third thermistor stalk -   617B third thermistor -   617C fourth thermistor stalk -   617D fourth thermistor -   618A first switch -   618B second switch -   618C third switch -   618D fourth switch -   619 bimetal hinge base retaining plate -   620 bimetal hinge base retaining plate nut -   700 top view of the sensor lever and switch actuator elements -   701 coil spring stud -   702 switch actuator element adjuster screws -   703 unused actuator element adjuster screw threaded hole -   704 switch actuator elements -   705 switch actuator elements flange screw -   706 bimetal hinge lever retaining plate -   707 bimetal hinge lever retaining plate nut -   708 bimetal hinge -   709A first switch PCB mounting boss -   709B second switch PCB mounting boss -   709C third switch PCB mounting boss -   709D fourth switch PCB mounting boss -   800 bottom view of switch PCB -   800A perspective view of switch PCB from bottom -   900A perspective view of sensor internal components from bottom -   1000 front view of sensor internal components -   1001 switch actuator element adjuster screw boss -   1000A front view of sensor internal components with riser cutaway -   1000B front view of lever displacement detail -   1002 bellows lever coupling -   1003 bellows -   1004 lever displacement dimension -   1004 lever displacement dimension -   1100 top view of sensor internal components -   1100A crosssection view from right side of sensor internal     components -   1101 gas port -   1102 sensor module -   1103 processor module -   1104 stabilizer -   1200 top view of sensor internal components -   1200A crosssection view from front of sensor internal components -   1300 hardware block diagram -   1301 MCU (microcontroller unit) -   1302 communication subsystem -   1303 analog to digital converter subsystem -   1304 digital to analog converter subsystem -   1305 digital I/O interface subsystem -   1306 safety limit detection subsystem -   1307 temperature interface -   1308A first temperature sensor -   1308B second temperature sensor -   1308C third temperature sensor -   1308D fourth temperature sensor -   1309 displacement transducer interface subsystem -   1310 high resolution displacement transducer -   1311 battery -   1312 network management controller -   1400 processing block diagram -   1401 raw conversions from displacement and temperature subsystems -   1402 signal to noise enhancement -   1403 displacement to pressure calculation -   1404 pressure calibration -   1405 digital filtering subsystem -   1406 signal processing parameter set -   1407 pressure readings output to gas mass calculations and user     interfaces -   1500 lever position as a function of gas pressure at 25 C -   1501 lever response from 0 psig to 60 psig, ball nose spring plunger     operating -   1502 lever response from 60 to 100 psig, coil spring and bimetal     hinge only -   1503 a particular lever position of 0.026″ -   1504 a pressure of 81 psig corresponds to position of 0.026″ -   1600 normalized sensor response as a function of lever position -   1601 response of reflective object sensor -   1602 response of Hall effect sensor -   1603 a particular HES sensor response of 0.55 -   1604 a particular lever position of 0.026″ corresponds to sensor     response of 0.55 -   1700 graph of pressure compensation required as a function of     temperature -   1701 pressure compensation required as a function of temperature -   1702 zero compensation required at reference temperature -   1703 a particular temperature -   1704 a particular pressure compensation corresponds to a particular     temperature -   1800 graph of temperature as a function of normalized temperature     sensor response 

The invention claimed is:
 1. A method for computing gas density comprising the steps of: acquiring a raw temperature measurement from a temperature probe in connection with a gas; converting said raw temperature measurement to an accurate temperature reading using pre-determined temperature calibration information; acquiring a raw displacement measurement from a displacement sensor in connection with a moveable element responding to the pressure and to the temperature of said gas; converting said raw displacement measurement to an accurate displacement measurement using pre-determined displacement calibration information; determining a nominal pressure measurement corresponding to said accurate displacement measurement using pre-determined displacement to pressure transformation information; converting said nominal pressure measurement to an accurate pressure reading using said accurate temperature reading in conjunction with pre-determined temperature to pressure compensation information, and computing the density of said gas using said accurate temperature reading and said accurate pressure reading in conjunction with a gas law.
 2. A method for computing gas density as claimed in claim 1 wherein said temperature probe is a thermistor.
 3. A method for computing gas density as claimed in claim 1 wherein said displacement sensor is a reflective object sensor.
 4. A method for computing gas density as claimed in claim 1 wherein said displacement sensor is a Hall effect sensor.
 5. A method for computing gas density as claimed in claim 1 wherein said gas is sulfur hexafluoride.
 6. A method for computing gas density as claimed in claim 1 where said gas law is a second or higher order polynomial function of said gas density and said polynomial function coefficients are a function of said accurate temperature reading.
 7. A method for computing gas density as claimed in claim 1 further comprising the step of: recording all measurements, readings, and calculations in a microcontroller memory including said raw temperature measurement, said accurate temperature reading, said raw displacement measurement, said nominal pressure measurement, said accurate pressure reading, and said gas density.
 8. A method for computing gas density as claimed in claim 7 further comprising the steps of: repeating said foregoing steps periodically thereby creating a stored time sequence of said measurements, readings, and gas density, and utilizing said stored time sequence to determine a leakage of gas or an addition of gas. 